Internal combustion engines such as engines mounted in motorcycles and other vehicles are commonly equipped with fuel injectors configured to inject an optimal amount of fuel to fresh air taken in from outside and flowing in an air-intake passage of the engine at optimal timings, for the purposes of improvement of an output power and fuel efficiency and of emitting a clean exhaust gas.
Such a fuel injector is configured to inject in each intake stroke an amount of the fuel according to a load state and a rotation state of the associated internal combustion engine. Actually, a part of the fuel injected from the fuel injector adheres onto a wall surface of the air-intake passage and is vaporized, and the vaporized fuel is fed to a cylinder as a fuel for either one of the next two intake strokes rather than for the corresponding intake stroke.
When the internal combustion engine is running in a steady running state under the above described condition, the amount of the fuel that adheres onto the wall surface and the amount of the fuel vaporized from the fuel adhering onto the wall surface are balanced so that an optimal amount of fuel is fed to the cylinder in each intake stroke.
However, when the engine is changing its running state, namely, in a transitional running state that is an acceleration state or a deceleration state, the amount of the fuel that is injected from the fuel injector and adheres onto the wall surface and the amount of the fuel vaporized from the fuel adhering to the wall surface do not match. This makes it difficult to obtain a desired acceleration or deceleration state or otherwise an optimal combustion state. To be specific, in the acceleration state, the amount of fuel that adheres onto the wall surface becomes more than the amount of fuel vaporized, and an air-fuel ratio is increased because of a lean air-fuel ratio. As a result, the desired acceleration state cannot be obtained. On the other hand, in the deceleration state, the air-fuel ratio is decreased because of an rich air-fuel ratio, making it difficult to obtain a clean exhaust gas and to obtain desired deceleration.
In the past, in order to obtain a correct air-fuel ratio even in the transitional running state, there has been known a fuel injection controller for controlling the fuel injector which is configured to execute compensation control depending on the condition of the transitional running state in such a manner that the amount of fuel to be injected is increased in the acceleration state of the internal combustion engine and is decreased in the deceleration state (see Japanese Patent Publication No. Sho. 62-101855).
In the above described compensation control, however, the fuel injection controller executes the compensation control based on a fuel injection amount calculation base map using as parameters, an engine speed, a throttle opening degree, and an air-intake pressure, to determine the amount of fuel to be injected in the corresponding intake stroke. The transitional running state, particularly the transitional running state including a large acceleration state or a large deceleration state, is varied significantly with time. As a result, a time lag in the compensation control occurs, making it difficult to feed the optimal amount of fuel at optimal timings.
The compensation control is executed with reference to the fuel injection amount calculation base map showing a complicated correlation between the engine speed, the throttle opening degree, the air-intake pressure, etc., from the steady running state to the transitional running state of the internal combustion engine. For this reason, if a control content (i.e., a controlled parameter) of the compensation control is compensated to eliminate the time lag in the acceleration state, then an exhaust gas may be changed into an unfavorable one in the deceleration or other states, or otherwise a driving feeling may become worse. Thus, it is difficult in many cases to compensate the controlled parameters.